Not every rock star has a species of tree frog - the Dendropsophus stingi - named after them, but then Sting has been pursuing his passion for the preservation of rainforests and their indigenous people for approaching 20 years.

Tomorrow night that landmark will be celebrated at New York's Carnegie Hall in the latest of Sting's concerts to raise money for the Rainforest Foundation he started with his wife, Trudie Styler, in 1989. With tickets selling for up to $600 (£300) each, the organisers hope to raise more than $2m for projects in Latin America, Africa and around the world.

Sting will be joined on stage by Feist, Billy Joel, the former Beach Boy Brian Wilson and other artists.

Alluding to its past victories - most notably the winning of legal recognition in 1993 for 37,000 sq miles of Amazonian territory for the Kayapo Indians - the event is being called Some Kinda Legacy.

But the 20th anniversary of Sting's activism has also invited closer scrutiny of that legacy. The Carnegie Hall gig has been overshadowed by criticism over the way the charity network is being managed.

Most of the censure relates to its New York-based US branch. Charity Navigator, which monitors more than 5,000 non-profit organisations in America, has given Rainforest Foundation US its lowest rating for three years running.

Based on assessments of how the financial assets are managed, the watchdog gave the US branch zero stars out of four for 2004, 2005 and 2006 - the latest year for which the assessment has been completed. (Less than 2% of the charities it monitors fall into the zero-star category.)

The watchdog's main criticism was that in 2006 the US foundation spent only 60% of its funds on actual programmes on the ground - compared with a sector norm of 75%. A watchdog spokeswoman said its records showed that the foundation did even worse in 2004 when 43% of its revenues were spent on such projects.

"One positive thing is that it is trending in the right direction, but they are still underperforming compared with their peers," said Sandra Miniutti. "It's not a new charity; it's been around a long time and we would expect to see greater efficiency."

Franca Sciuto - the chair of the parent body, Rainforest Foundation Fund, and one of the founding members along with Sting and Styler, who both remain on its board - said the US branch had been through recent difficulties. She said a couple of directors had been sacked and the charity had been put in order, with its records in 2007 showing marked improvements.

Despite these local difficulties, Sciuto said, the overall legacy of the foundation had been highly successful. Over the 20 years, some $25m had been raised; of that 85% had gone to projects for native land rights and forest preservation.

· This article was amended on Saturday May 17 2008. In this article we began to refer to the Rainforest Foundation US as the Rainbow Foundation halfway through. The Rainbow Foundation is a children's charity; it has no connection with the Rainforest Foundation. This has been corrected.